chore: add php-cs-fixer with PSR-12 config and CI check

- Add friendsofphp/php-cs-fixer to require-dev
- Create .php-cs-fixer.dist.php configured for @PSR12 ruleset
- Add php-cs-fixer dry-run step to CI pipeline
- Auto-fix existing PSR-12 violations
- Document code style tooling in readme.md

src/Service/DomainInterface.php

@@ -2,7 +2,8 @@
 
 namespace App\Service;
 
-interface DomainInterface {
+interface DomainInterface
+{
     /** IE: "auth.example.com" or null if not using a separate subdomain
      * @return ?string Returns auth subdomain if configured, otherwise null */
     public function getAuthSubdomain(): ?string;